A controller area network (CAN) has become widespread as an in-vehicle network. Equipment for in-vehicle use is strongly required to prevent malfunctions. It is therefore necessary to protect equipment connected to the CAN sufficiently against anomalous attacks via the CAN. For example, there is a method of detecting anomalous messages by checking periodicity of messages on the CAN (see, for example, PTL 1). This detection method is required to prevent a legitimate message from being erroneously detected as an anomalous message because of a slight shift in transmission timing of the legitimate message or a slight transmission delay. It is therefore conceivable that a certain margin is provided for a determination parameter.